Effect of Presodiation Additive on Structural and Interfacial Stability of Hard Carbon | P2‐Na _0.66 Mn _0.75 Ni _0.2 Mg _0.05 O ₂ Full Cell

Abstract The compatibility between a corncob‐derived hard carbon anode and a layered oxide cathode (Na _0.66 Mn _0.75 Ni _0.2 Mg _0.05 O ₂ ), as well as the effect of presodiation via cathode additive (Na ₂ C ₄ O ₄ ), are investigated in a sodium‐ion full cell. Extensive physicochemical and electrochemical characterizations are performed to deeply investigate the structural and interfacial evolution of the materials in the presodiated cell, either within the single cycle or upon long‐term cycling. The use of sacrificial cathode additives is generally regarded as a method to improve full cell performance, especially when using sodium‐deficient materials. However, undesired effects upon decomposition of the sacrificial salt may arise due to the complexity of the system. In this study, it is evidenced that the presodiation changes the properties of the electrodes causing a worsening of the electrochemical performance of the cell during long‐term cycling. The surface morphology of the cathode is negatively affected by the formation of holes/cracks, while redox processes associated with structural transformations are suppressed in the voltage window of interest, with partial structural deformations occurring during activation cycles. The SEI and CEI are also affected by the formation of insulating organic species, which lead to an increased thickness and interphase resistance hampering the charge transfer kinetics of the cell.